This invention relates to a noise shaping circuit and a noise shaping method for feeding the quantization noise generated by a quantizer via a noise filter back to an input side of the quantizer.
In general, in a noise shaping circuit, the quantization noise generated by the quantizer is fed back via the noise filter to the input side of the quantizer for changing the frequency characteristics of the quantization noise. The noise shaping circuit may be thought of as belonging to a digital filter having a feedback loop.
Such noise shaping circuit may be occasionally utilized for improving the sound quality of the sound recorded with 16 bits to the level as if the source was recorded with e.g. 20 or 24 bits, without changing the format for the compact disc (CD) or the digital audio recorder (DAT) providing that the number of bits of sample data equal to 16 as disclosed for example in U.S. Pat. No. 5204677.
That is, in converting source signals recorded with 20 or 24 bits into 16-bit signals, the frequency characteristics of the quantization noise are modified by way of noise shaping for approximating the frequency characteristics of the quantization noise to the loudness equalization curves, in order to take account of auditory characteristics of human ears, above all, equal loudness curve characteristics, for reproducing the music with the rich and ambient feeling having reduced harmonic distortions and improved input/output linearity.
Meanwhile, with the above-described digital filter in the feedback loop, it occurs frequently that oscillation is produced due to the so-called limit cycle even although the input signal becomes zero or is of a constant value, such as a dc value. That is, when the input signal should be a dc value, the data left in the filter continues to be oscillate to output oscillation data by way of hunting. If such hunting is produced, an output value may be replaced by a substitute value or the output may be decreased on detection of the hunting. In such case, it is necessary to provide an additional operating step. On the other hand, an output other than a zero output occasionally cannot be obtained for an input dc value.
Also, in the noise shaping circuit in which the quantization noise is fed back via a noise filter to an input side of the quantizer, oscillation data due to hunting tends to be added to the input data even though the number of bits of the input data is equal to or less than the number of bits of the output data.